1. Field of the Invention
This invention relates generally to the field of disc drive data storage devices and more particularly, but not by way of limitation, to an improved pivot bearing structure for use in a rotary actuator in a disc drive data storage device.
2. Brief Description of the Prior Art
Disc drives of the type referred to as "Winchester" disc drives are well known in the industry. Such disc drives incorporate a "stack" of one or more disc-shaped platters mounted on a spindle motor for constant high speed rotation. The surface of each of these discs is coated with a magnetizable medium for recording digital data in a plurality of circular, concentric data tracks.
A number of read/write heads act in cooperation with the disc surfaces for the recording and retrieval of data. These heads are attached to some sort of actuator mechanism which operates under the control of electronic circuitry to controllably move the heads from track to track
In the archetype "Winchester" disc drive, these heads rest in contact with the surface of the discs prior to the application of power to the drive. As the spindle motor receives power and begins to spin, the discs drag a thin layer of air along with them. As the discs accelerate toward their specified operating speed, this thin layer of air interacts with ski-like structures on the surface of the read/write heads and the heads begin to "fly" above the disc surface. The height at which the heads "fly" above the disc surface, or "flying height", is dependent upon the geometry of the head, the head support means and the linear velocity of the disc under the head, along with other factors.
The actuator that moves the read/write heads has taken many different forms over the years. Early Winchester disc drives for personal computers used a stepper motor to move the read/write heads in a straight radial line across the disc surface. This radial movement was guided by an arrangement of parallel rods and ball bearings and is referred to as a "linear stepper actuator". A similar arrangement of guide rods and ball bearings powered by a voice coil motor (VCM) was also frequently used and is called a "linear VCM actuator".
A second type of actuator that has often been used is the rotary actuator. A rotary actuator, as its name implies, includes a pivot mechanism located closely adjacent the outer diameter of the discs, and has an axis of rotation that is parallel to the spin axis of the spindle motor and discs. A number of head support arms extend from the pivot mechanism over the surfaces of the discs and permit the heads to move in an arcuate path from track to track. Again, both stepper motors and VCMs have been used to power the head movement, and actuators using these motors are referred to as "rotary stepper actuators" and "rotary VCM actuators", respectively.
Rotary actuators are generally more compact than linear actuators and have thus found favor in recent disc drive designs for the "three-and-a-half-inch" and "two-and-a-half-inch" form factors. Another advantage of rotary actuators is that they usually have a smaller moving mass than comparable linear actuators, and, as such, provide faster head movement, causing faster data access.
No matter what type of motor is used to power a rotary actuator, the precision of the pivot mechanism is crucial to the proper operation of the disc drive. A typical rotary actuator pivot mechanism includes a pair of ball bearing assemblies situated one above the other with the inner races of the bearings fixed to a stationary shaft and the outer races attached to a structure that includes the driving motor, or attachment thereto, and the head support apparatus. The stationary shaft is commonly fabricated from steel and either screwed or staked into the base casting of the disc drive unit in as precise a relationship to the spindle motor and discs as is permitted by manufacturing technology. The structure that supports the heads and attaches to the actuator motor is typically made from aluminum or magnesium to minimize the moving mass. The bearing assemblies themselves are made of steel. This mixing of materials, steel for strength and wear resistance and aluminum or magnesium for lightness, leads to several of the principal engineering challenges involved in the design of rotary actuators.
The first of these engineering challenges involves the effects of differential thermal expansion. Since steel and aluminum or magnesium have significantly different coefficients of thermal expansion, i.e., they expand and contract at different rates and to different extents for the same temperature change, changes in the geometric relationship of these critical parts can be altered by changes in temperature. Disc drives of the current technology are typically specified to operate over an ambient temperature range of 5.degree.-50.degree. C. (41.degree.-122.degree. F.), and the internal temperature can be expected to rise another 20.degree. C. above ambient. This temperature range is wide enough to cause off-track problems in the disc drive unless carefully considered by the designer.
The second problem caused by the use of differing materials lies in how the various components are attached to each other. This attachment is commonly accomplished by either press-fitting or the use of adhesives. Each of these techniques creates its own set of potential problems which will be discussed in more detail below.
Another consideration in the design of a rotary actuator is the resonant frequency of the assembly. Manufacturers of computer systems specify the amount and type of shock and vibration that the disc drive must be able to endure without suffering damage and without degrading data handling operations. Since the actuator of a disc drive must move the heads at a high rate of speed, a faulty design can result in sympathetic vibrations being induced in the actuator mechanism, inhibiting the ability of the disc drive to accurately read or recover data. In a properly designed actuator, the natural frequency should be high enough to ensure that such sympathetic vibrations do not occur.
Clearly a need exists for an improved pivot mechanism that minimizes the size, and thus the moving mass, of the rotary actuator, while overcoming the traditional problems encountered when creating a pivot mechanism out of differing materials.